Following the explosion, ambulances rushed to the site in Sultanahmet square, close to the Blue Mosque and Hagia Sophia, in a major tourist area of Turkey’s most populous city, ferrying away the wounded as police cordoned off streets.

Police are reportedly taking extra measures to protect people against the possibility of a second explosion.

Al Jazeera’s Emre Rende, reporting from Istanbul, said information was scarce immediately after the blast.

“Witnesses have said that the blast was heard from other neighbourhoods,” he said. “Witnesses said that the ground shook.”

Rende said police were conducting searches outside of the cordoned off area, in case a second bomber was involved.

Erdem Koroglu, who was working at a nearby office at the time of the explosion, told NTV television he saw several people lying on the ground following the blast.

“It was difficult to say who was alive or dead,” Koroglu said. “Buildings rattled from the force of the explosion.”

The blast comes just over a year after a female suicide bomber blew herself up at a police station for tourists off the same square, killing one officer and wounding another.

Turkey has become a target for the Islamic State of Iraq and the Levant (ISIL), with two bombings last year blamed on the armed group, in the town of Suruc near the Syrian border and in the capital Ankara. The latter killed more than 100 people.

Violence has also escalated in the mainly Kurdish southeast since a two-year ceasefire collapsed in July between the state and the Kurdistan Workers Party (PKK) armed group, which has been fighting for three decades for Kurdish autonomy. (Al Jazeera)